1. Field of the Invention
This invention relates to semiconductor packaging in general, and in particular, to a method of producing a low-cost printed circuit board (xe2x80x9cPCBxe2x80x9d) for a semiconductor package that has an integral heat sink capable of conducting away relatively large amounts of heat generated by components in the package during operation.
2. Description of the Related Art
The recent trend in consumer electronics has been toward smaller, lighter products having expanded functional capabilities and capacities. This trend has, in turn, resulted in a demand for packaged semiconductor devices that are smaller, yet more highly integrated and of higher capacity. Accordingly, packaging candidates for modern semiconductor devices must not only possess excellent electrical characteristics, large input/output-terminal capacities, and high heat dissipating capabilities, but must also provide these features at a competitive price if they are to remain commercially viable in this very cost-competitive environment.
One such low-cost packaging candidate is the ball grid array (xe2x80x9cBGAxe2x80x9d) semiconductor package. BGA packages are easily formed on a conventional PCB and can effectively reduce the overall length of electronic circuits incorporating them. BGA packages also utilize power- and/or ground-bonding areas more effectively, thus yielding excellent electrical characteristics. The input/output terminal density of BGA packages is greater than that of conventional quad flat packages (QFPs), which better comports with the trend toward smaller, denser packages. And, for semiconductor chips dissipating relatively low amounts of heat during operation, BGA packages have relatively good heat dissipating characteristics.
However, the new, high-speed, high-power-dissipating chips require even higher heat dissipating capabilities in their packaging. Several methods have been proposed to enhance this capability. These typically involve mounting the chip(s), directly or indirectly, on a heat sink, or xe2x80x9cslug,xe2x80x9d that is laminated or soldered to the top or bottom side of an interconnection substrate, such as a PCB.
For example, M. Hundt, et al. of SGS-Thomson Microelectronics, Inc., in a paper presented at the xe2x80x9cxe2x80x295Flip Chip, BAG, TAB and AP Symposium,xe2x80x9d entitled, xe2x80x9cConduction-Cooled Ball Grid Array,xe2x80x9d ((copyright)1995 Semiconductor Technology Center, Inc.), describe a BGA package in which a copper heat sink, or xe2x80x9cslug,xe2x80x9d is laminated to the bottom surface, i.e., the surface on which the solder balls are mounted, of a PCB having a rectangular opening in it. A microchip is epoxied to the slug in the opening and is wire-bonded to the surrounding PCB substrate to effect electrical interconnection of the chip. The bottom surface of the heat slug is, in turn, soldered to a multilayer main board having a relatively thick ground plane. Plated-through via holes conduct heat from the bottom of the heat slug to the ground plane to convey heat away from the chip. The authors claim that this design reduces the internal thermal resistance (xcex8JC) of the package to a value typically less than 1 degree C/Watt for most sizes of chip.
A somewhat similar arrangement, in the context of a xe2x80x9ctotal encasementxe2x80x9d chip carrier package (xe2x80x9cTExe2x80x9d package), is described in U.S. Pat. No. 5,650,593 to J.R. McMillan, et al. This reference describes several embodiments of a xe2x80x9cthermally enhancedxe2x80x9d package, one of which includes a circuit substrate with a center opening and a solder-plated metal ring attached to the bottom surface of the substrate and surrounding the opening. A heat sink is soldered to the metal ring such that a portion of it is exposed through the opening, and one or more microchips are epoxied to the exposed surface. A plastic or a metal ring also surrounds the opening on the top surface of the substrate to define a cavity, and a metal or plastic lid attaches to the ring to close off the cavity with a xe2x80x9cgas tightxe2x80x9d seal.
Another laminated heat slug arrangement is described in U.S. Pat. No. 5,734,555 to J. F. McMahon, in the context of a plastic pin grid array (xe2x80x9cPPGAxe2x80x9d) semiconductor package. In this package, a microchip is electrically connected to a multilayer, intermediate PCB by inverting the chip relative to the board and contacting it to the board so that interconnection pads on the top surface of the chip engage corresponding connection pads on the bottom surface of the PCB (the so-called xe2x80x9cflip-chipxe2x80x9d method). The PCB has a step in it to receive the chip, and a central opening through it that exposes the top surface of the chip through the board. A copper heat slug with a rabbeted face is attached to the top surface of the board, with the rabbeted face disposed in the opening above the chip and bonded to it with a layer of thermally conductive epoxy. The heat slug may also be attached to a finned heat sink on the top of the package for enhanced convective-air cooling.
Yet another laminated heat sink arrangement in the context of a plastic molded package having a lead frame is described in U.S. Pat. No. 5,455,462 to R.C. Marrs. The heat sink in this reference features a circumferential xe2x80x9clocking ringxe2x80x9d that engages and keys with the plastic encapsulating the package to provide a better seal between the encapsulant and the heat sink.
A somewhat more radical approach to cooling of very high heat dissipating microchips is described in U.S. Pat. No. 5,365,400 to T. N. Ashiwake, et al. Here, one or more bare semiconductor chips are mounted to a ceramic main board using the flip-chip method, and a heat sink is soldered directly to the top surface of each of the bare chips. The heat sinks, which are individually supplied with a forced cooling fluid, e.g., fluorocarbon, may be connected to a plenum, or header, by means of an extensible bellows.
While each of the foregoing solutions addresses the problem of enhanced microchip cooling to a greater or lesser extent, they do not address the problem of achieving this result in a simple, low-cost packaging arrangement. In particular, it may be seen that, in those references that utilize a conventional PCB to interconnect the microchip, one or more manual, and in some cases, relatively complex, post-PCB-lamination fabrication and/or assembly steps are required to implement an integral heat sink into the package. These additional steps necessarily result in additional costs to the package, which detracts somewhat from their desirability for use in a consumer electronics commercial environment.
What is needed, then, is a lower-cost, easier-to-produce semiconductor packaging arrangement that achieves enhanced chip cooling without the need for any post-lamination procedures to implement a heat sink. Indeed, what is needed is a package that meets a xe2x80x9c5-watts-for-less-than-$5xe2x80x9d goal.
This invention provides a method for producing a low-cost PCB for a semiconductor package, e.g., a BGA semiconductor package, that incorporates a heat sink without the need for additional post-lamination assembly procedures, one that is capable of meeting the above, xe2x80x9c5-watts-for-less-than-$5xe2x80x9d goal. The novel PCB has an integral heat sink for the mounting of one or more semiconductor chips thereon, and in most applications, is capable of effectively conducting a relatively large amount of heat away from the package, an amount that is well in excess of that conveyed by conventional semiconductor package PCBs.
The method comprises punching an opening of a given size and shape through the thickness of a glass reinforcement that has been impregnated with a B-stage epoxy resin. A heat slug of a thermally conductive metal having about the same size and shape as the opening is inserted into the opening, and the slug containing substrate is then sandwiched between two sheets of an electrically conductive metal, e.g., copper foil. The sandwich is placed in a heated press, which applies heat and pressure to the two opposite faces of the sandwich, forcing the resin to melt and flow into the spaces in the opening between the opposing sidewalls of the slug and the substrate, where it solidifies, thereby simultaneously bonding the slug to the substrate within the opening, and adhering the conductive layers to the upper and lower surfaces of the substrate, with the heat slug sandwiched therebetween.
Subsequent processing of the PCB is the same as with conventional PCBs, and may comprise removing, e.g., by etching away, of portions of the conductive layers overlying the slug on opposite sides of the substrate to expose a semiconductor die-bonding pad directly atop the slug, as well as an area for thermally bonding the slug, e.g., with solder or a filled epoxy, to a main board on the bottom of the slug.
A better understanding of these and other features and advantages of the method may be obtained from a consideration of the detailed description of the invention below, particularly if that description is considered in conjunction with the accompanying drawings. Following is a brief description of the several views of those drawings.